1. Field of the Invention
The present invention relates to a method and an arrangement for tensioning a staggered anchorage.
2. Description of the Background Art
Pressure-grouted anchorages are known, for example, as ground or rock anchorages. They are generally comprised of a plurality of axis-parallel tension members of steel rods, steel wires, or steel wire strands, which are guided into a bore hole. By grouting at the furthest end of the bore hole, a grouted body is formed, which bonds the tension members with the surrounding ground for transmitting a load to the underground. The longitudinal segment of a tension member, which facilitates load transfer, is referred to as an anchorage length Ltb. At their opposite end, the tension members are anchored, with the aid of anchorage wedges, in an anchorage disk, which rests on an above-ground bore hole end. During the tensioning of the pressure-grouted anchorage, the tension members in the area between the anchorage disk and the grouted body can elongate freely. Therefore, this area is also referred to as a free steel length Ltf.
A staggered anchorage is a special embodiment of a pressure-grouted anchorage, wherein the load transmission area is not concentrated at an end of the pressure-grouted anchorage, but instead is distributed over a larger longitudinal section of the pressure-grouted anchorage. By distributing the anchorage force over an extended load transmission area, a more balanced loading into the underground takes place, thus improving the anchorage effect. The distribution of the load is achieved by utilizing tension members of varying length, the ends of which terminate at various bore hole depths. The result thereof is an axial staggering of an anchorage length Ltb in the bore hole.
When tensioning a pressure-grouted anchorage, industrial standards require that, for security reasons, the tension members are tensioned to a defined test load Fp before subsequently being impacted, by repeated de-tensioning and re-tensioning, with the required working load. For the tensioning operation, it is common for pressure-grouted anchorages with tension members of identical length to use a multistrand jack, whereby with one hoist of the jack, all tension members are elongated simultaneously and to the same extent. Thus, all tension members are in the same state of tension during the tensioning process.
In contrast, the problem with tensioning staggered anchorage is that with uniform elongation of all tension members, varying states of tension would occur due to their different free steel lengths Ltf. Shorter tension members would be subjected to more stress as compared to longer tension members so that in shorter tension members, the test load Fp would already be reached at an elongation, at which longer tension members would still be far below the test load Fp.
For this reason, staggered anchorages are tensioned with hydraulically interconnected monojacks, that is, there is one dedicated jack for each tension member, which tensions the tension member until the test load Fp is reached. As a result of the varying free steel lengths Ltf of the tension members, different elongation values are obtained. Once the test load Fp is reached, the individual tension members are adjusted to a uniform working load, that is, after the tensioning operation is completed, all tension members, regardless of their length, have the same working load.
The necessity to have on hand and to operate multiple monojacks, has proven to be extremely costly, both technically and economically. In addition, using multiple monojacks entails considerable expenditures for the required measuring and logging labor. Although, from a technical viewpoint, applying a uniform working load to the individual tension members helps achieve a high anchorage force, however, it has the disadvantage that in the event of unexpected elongation of the anchorage, for example, due to deformations below ground, the elongation reserves of the individual tension members are different. With tension members of shorter free steel lengths, the reserves will be used up after a short overelongation, thus running the risk that these tension members fail.